In nuclear power plants and other critical locations, instruments are installed which record potentially-damaging motion of the earth, as from earthquakes. A typical known such instrument is a three-axis strong-motion accelerograph, which records the acceleration components of the earth motion with, e.g., a constant sensitivity of the order of 1 g full-scale over a frequency range of 0-20 Hz. Such instruments record on moving film or tape and require electrical power.
To back up such devices, simpler instruments are also employed. These yield only the peak values of the accelerations but are entirely passive, requiring no power and no maintenance, so that they may stand by reliably for years without attention. Once such peak accelerograph comprises a set of damped seismic mass-spring systems mounted on mutually perpendicular axes, each adapted to displace a small permanent magnet across the surface of a small stationary slip of magnetic tape. After a seismic event has occurred, the tape slips may be removed, and the lengths of the "tracks" traversed by the small magnets determined. Such slips of magnetic tape are, however, subject to erroneous recording while being inserted or removed and to erasure by strong magnetic fields, and will not survive a fire.